Mapping of the worst oil pollution distribution is based on the above steps. The scene on which most slicks have been detected in a frame is then selected for re-examination. All detected oil slicks on this image are further classified into either high or low probability ones. A similar normalised oil slick index for either total or high probability slicks can be calculated for this frame as:

where N is the detected slick number of the scene for any of the total, high & low probability slicks, f_sea is the sea surface fraction of the frame, and f is the scene fraction. These steps are repeated until all the included ERS frames have been completed so that the spatial distribution of oil pollution distribution in the region can be derived.

Spatial Distribution of the Average Oil Slick Occurrence Intensity
The overall statistical result for the average oil slick occurrence intensity is summarised in Table 1. A total of 10988 oil slicks have been detected in 2292 scenes, representing 45.6% of the total scenes studied. Spatially speaking, over 80% of the total study area had been once polluted at an intensity of at least one slick per 10,000 km² of sea surface area during the study period. Linear and patchy slicks have an almost equal frequency (45%) with curved ones much lower (10%). More than 10% of the oil slicks are over 10 km in size and over a quarter below 1 km in size, while about 60% of the total detected slicks fall in the range of 1-10 km.
Table 1. Statistical results of the detected oil slicks in the Southeast Asian waters during the period of September 1995 to September 1998.

Detected slicks		Number	Percentage (%)
Total		10988	100.0
Slick types	Linear	5045	45.9
	Curved	1175	10.7
	Patchy	4768	43.4
Slick sizes	³10km	1338	12.2
	5-10km	1824	16.6
	1-5km	4695	42.7
	<1km	3131	28.5
Total scenes		5029	100.0
Total frames		657	100.0
Polluted scenes		2292	45.6
Polluted frames		552	84.0

The normalised slick intensity is summarised in Table 2. The averaged oil pollution intensity over the whole study area is 2.7 with the highest of 28.8. The normalised slick intensity is then used to compile the spatial distribution map of the average ocean oil slick occurrence intensity in this region (Figure 2).
Table 2. Statistical results of the normalized average oil slick occurrence intensity in the Southeast Asian waters during the period of September 1995 to September 1998.

Intensity	Total	Slick type			Slick size (km)
		Linear	Curved	Patchy	³10	5-10	1-5	<1
Maximum	28.8	16.7	6.0	13.3	10.6	5.5	13.3	8.4
Average	2.7	1.2	0.3	1.2	0.3	0.4	1.1	0.8

Figure 2. Spatial distribution map of the average oil pollution occurrence intensity in the Southeast Asian waters.